The Treekeepers

"Liska lives in Arborven, a city surrounding an extraordinary tree that gives all those living there special powers. As a shapeshifter, Liska is training as a warrior. When she discovers that the Tree is under threat, it is her duty to act - but she can't convince anyone to listen to her. So with Lug, whose power over earthworms is dismissed as useless, and a ghost-girl, Elowen, she goes on an epic journey to defeat the worst threat their world has ever known." (Amazon UK)